Add ExternalPrerenderRequestHandler and related API

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index 954edb55bb6d2bfff58f8189336f8b0ef0ab520a..5ca529485fda81c6bb9037e9f2f055bb2a2c7384 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -392,6 +392,15 @@ PrerenderHandle* PrerenderManager::AddPrerenderFromLocalPredictor(
                       size, session_storage_namespace);
 }
 
+PrerenderHandle* PrerenderManager::AddPrerenderFromExternalRequest(
+    const GURL& url,
+    const content::Referrer& referrer,
+    SessionStorageNamespace* session_storage_namespace,
+    const gfx::Size& size) {
+  return AddPrerender(ORIGIN_EXTERNAL_REQUEST, -1, url, referrer, size,
+                      session_storage_namespace);
+}
+
 void PrerenderManager::DestroyPrerenderForRenderView(
     int process_id, int view_id, FinalStatus final_status) {
   DCHECK(CalledOnValidThread());